Just a shortish (relatively short anyway, like how Jupiter is relatively short compared to the Sun) bulletpoint list of common gameplay features and why I like them, despite all three being used in practically every game (of the relevant genre) out there.
* Collection Subquests: These started out as the most benign and dull of all RPG subquests, where you were charged to simply recover some number of otherwise useless objects from the wizard's dungeon or some such. However, when the same feature is transplanted into a different genre such as a 3D Platformer it becomes... even more pointless, since there's usually even less of a reward waiting for you upon completion. However, it does a good job of extending the lifespan of a game artificially after the fact (sort of like a Terri Schiavo effect) and it allows for some really devious optional gameplay to be added by designers, such as requiring the completion of an extremely difficult subgame for the capricious goal of some random mcguffin, just so you can have a finished collection. I can't even count the amount of times my stubbornness to get an optional item collection finished has allowed me to perform superhuman feats of gamer skill. So for that I thank it. Or spurn it until the day I die for wasting so much energy on it, whichever.
On a side-note, the grossly underappreciated PC game Anachronox had its own item collection item called a TACO: Totally Arbitrary Collectable Object. So if I ever mention the use of TACOs in the future you'll know what I mean.
* Optional "Super" Bosses: Similar to the above, but much more focused around the RPG genre, the ideology of a super-boss is to give the "hardcore" player a greater challenge after the game is complete with a much bigger boss than the one you usually find at the end of the game's story. You'd be hard-pressed to find a RPG made after the SNES/Genesis era that didn't have at least one of these, but again even though this has become a somewhat obnoxious and ubiquitous feature, it still allows for some interesting game design.
Take, for example, your average RPG: Final Fantasy 8 is a perfect example for sheer averageness, so we'll use that. Final Fantasy often relies on the reputation of their "Weapon" bosses - huge dragon/demon hybrids with more HP than God (sometimes literally depending on the end boss) - as the greatest challenge a FF game has. In the final dungeon of the game, a series of events you can perform will allow you to fight Omega Weapon, a battle far tougher than the usual final medley of grotesqueries with semi-philosophical roots that you'll come across at the end of these games. This is just a simple instance of the optional boss feature.
However, with Final Fantasy X (in case people think I don't like the series), they've increased the whole optional boss scenario to a crazy zenith. There are now three whole subquests full of optional bosses, most notably the monster trainer place (where the right amount of captured creatures allows you to fight the uberdifficult "perfect" specimen of that particular monster class) and the Dark Aeons, which are all your summoning titan/god things made to look all gothic and hideously powerful. Finding these optional bosses tends to be almost as fun as getting your ass kicked by them (if you can manage anything as fun as that), and designers love adding easter eggs with these optionals. The blade-spinning uberboss in Kingdom Hearts 1 for instance was named after a lucky competition winner Kurt Zisa. Which again proves that even though a game feature can be overexposed, it may not be completely devoid of creative applications by a smart designer.
* Randomized Dungeons: Last one for today, methinks. Randomized dungeons are another of these groan-inspiring features that inundate the RPG genre these days, though it is in fact as old as dirt. The simplest way to create an endless combination of dungeons to fight through is to have your game's coding randomly create them, keeping such necessary elements as treasure, enemies, and a clear route between the entrance and exit. My two favorite games of all-time both used random dungeons to some degree (more on those two in a future update) and several games, if they don't feature them prominently in the game, have an "optional dungeon" (see optional bosses above also, since there's usually one at the end of these optional dungeons).
So you have some idea of the general application of such a feature. Now to move onto the more creative facilitations of it: Take Nippon-Ichi's series of Strategy RPGs (La Pucelle, Disgaea, Phantom Brave and Makai Kingdoms are the ones currently out in the States/Europe). The storyline is spread over 100 or so pre-generated maps, with their own little puzzles to solve and pre-set teams of enemies to beat. These maps make up approximately 1% of the gamer's time spent on that game (on average), as they're far more likely to be spending it in the optional randomized dungeons and levelling up for ungodly stats and equipment. The optional dungeons of Wild Arms 3 and Final Fantasy X-2 have 100 floors each. The optional dungeon of Wizardry: Tale of the Forsaken Land is purportedly endless. It dawns on one that a simple feature like this (as well as creating enough high-level equipment for your character to survive the later floors) can almost make a game's lifespan infinite, or at least increase its longevity exponentially. Putting all the best gear the game has to offer on the final few floors will entice enough players to attempt the ordeal, as long as you (as a designer) don't make it too painful for them (like removing the ability to save every so many floors, which is a killing blow for any optional dungeon which I've met way too often).
Long update again. But once I get talking on those three above things I'm unlikely to stop. I want to reiterate here though that I'm well aware all three of the above have been used so lazily or incompetently (far more often than competently, in fact) that they may very well turn off some of the more casual players, but for those who want to stick with a game they've enjoyed immensely those three features are often a godsend for extending your time with them. Like finishing a book you really enjoyed and finding it has three more optional chapters. Yay!
